Abstract LB171: The differentiated CD47 monoclonal antibody AO-176 exhibits significant in vivo activity against xenograft models of pediatric acute lymphoblastic leukemia (ALL)

Abstract

Abstract Introduction: CD47 is a cell surface marker that interacts with signal regulatory protein alpha (SIRPα) on macrophages and dendritic cells to negatively regulate phagocytosis. This interaction elicits a “don't eat me” signal preventing phagocytosis by immune cells, enabling cell survival. CD47 is expressed on tumor cells, including acute leukemia cells. AO-176 is a highly differentiated monoclonal antibody that targets CD47, exerts potent in vivo activity in mouse xenograft models, has an established safety profile in cynomolgus monkeys, and is being evaluated in clinical trials. Given the broad expression of CD47 in malignant hematopoietic cells, it was of interest for the Pediatric Preclinical Testing Consortium to evaluate AO-176 in vivo against patient-derived xenograft (PDX) models of pediatric ALL. Methods: CD47 mRNA and protein expression were quantified by RNAseq (https://pedcbioportal.org) and flow cytometry (relative fluorescence intensity, RFI), respectively. AO-176 was tested in vivo against 3 T-ALL (ALL-8, ALL-30, ALL-121) and one early T-cell precursor (ETP) ALL (ETP-2) PDXs in NSG mice at 25 mg/kg intraperitoneally once per week for 4 weeks. Engraftment was evaluated by enumerating the proportion of human CD45+ cells in the peripheral blood (%huCD45+) at weekly intervals. Events were defined as the %huCD45+ ≥ 25%. At Day 28 post-treatment initiation or event (whichever occurred first), 3 mice/group were humanely killed to assess leukemia infiltration of hematolymphoid organs. Drug efficacy was assessed by event-free survival (EFS) of treated (T) and control (C) groups by T-C, T/C, and stringent objective response measures (Houghton et al, Pediatr Blood Cancer 49:928-40, 2007). Results: CD47 mRNA expression was significantly higher in a panel of T-lineage ALL PDXs (n=25; mean + SD FPKM, 149 + 57.9) compared with B-ALL PDXs (n=65; 65.3 + 18.7 FPKM) (P<0.0001, unpaired t-test). Cell surface CD47 expression in a subset of 8 PDXs (6 T-ALL, 2 ETP-ALL) was a median of 40.2 (RFI range 17.9-81.9). AO-176 was well tolerated in NSG mice and significantly (P<0.006) delayed the progression of 3/4 PDXs tested (3 T-ALL, one ETP-ALL) (T-C, 12.7-56.9 days; T/C 3.4-4.9) and one PDX elicited an objective response (Partial Response). AO-176 was least effective against the lowest CD47 expressing PDX. AO-176 significantly (P<0.005) decreased human leukemia infiltration of murine spleens, but not bone marrow, in 3/4 PDXs. Conclusion: Despite being tested in highly immune-deficient mice, the naked antibody AO-176 exhibited significant single-agent in vivo anti-leukemic activity against pediatric T-lineage ALL PDXs. The significant decrease in spleen infiltration elicited by AO-176 in 3/4 PDXs suggests on-target activity consistent with the known mechanism of action of CD47 targeting agents. (Supported by NCI Grants CA199000 and CA199922) Citation Format: Richard B. Lock, Kathryn Evans, Narimanne El-Zein, Eric J. Earley, Stephen W. Erickson, Beverly A. Teicher, Victoria Sung, Malcolm A. Smith. The differentiated CD47 monoclonal antibody AO-176 exhibits significant in vivo activity against xenograft models of pediatric acute lymphoblastic leukemia (ALL) [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr LB171.